1. Field of Invention
The present invention relates to a single-pixel driver and, in particular, to an organic electroluminescence matrix-type single-pixel driver.
2. Related Art
The organic electroluminescence (OEL) structure usually consists of a glass substrate, a transparent indium-tin-oxide (ITO) anode, HTLandEML, and a metal cathode. When a voltage is imposed on such an OEL display, electrons and holes flow into the HTLandEML through the anode and the cathode, respectively. The annihilation of electrons and holes produces excitons and radiate photons. The OEL displays can be roughly classified into two different systems according to the material. The molecule-based device using dye or color materials is called an organic light-emitting diode (OLED), and the polymer-based device using conjugate polymers is called a polymer light-emitting diode (PLED). OEL displays have many advantages such as self-luminescence, back-light source free, high illumination efficiencies, low operation voltages, quick responses, no view angle limitations, wide operation temperature ranges, low power consumption, low manufacturing costs, being able to produce true colors, and extremely small thickness. They satisfy all the requirements for multimedia and will be the most favorable devices for modern displays.
Recently, due to the need in high resolutions in display panels, the pixel rate also increases. OLED devices 10, however, are limited by its material characters and parasite capacitance and thus cannot readily turn off pixels when the operation frequency increases accordingly (around 50 KHz). As shown in FIG. 1, VEE can connect to a low potential or negative pulse. A scan line 20 provides scan signals and a data line 30 controls the switch of transistors 40 so as to make the OLED device 10 emit light. The brightness can be further changed by adjusting the pulse width and amplitude imposed on the data line 30. Its drawback is that when the operation frequencies of both the scan line 20 and the data line 30 increase, the charge/discharge time is greater than the width of the pulse because of the OLED parasite capacitance effect. Thus, some pixels cannot become dark readily; that is, the OLED devices cannot easily turn off the pixels. For a conventional circuit as shown in FIG. 1A, where the transistor 40 is replaced by an NPN transistor 41, the OLED device still cannot readily turn off the pixel.
Accordingly, designing an OLED driver that can increase the operation frequency of the OLED and at the same time satisfy the requirements for high resolutions has become an important subject.
It is a primary objective of the present invention to provide a single-pixel driver, whose driving method is to use a transistor to control and accelerate the charge/discharge work speed of OLED devices so as to reach the needed work frequency (1 MHz).
The present invention adds a bypass transistor for discharging in a conventional driver so as to solve the response delay due to the parasite capacitance effect and to speed up charge removal. The circuit includes at least: an organic electroluminescence (OEL) device, a first transistor, and a second transistor. The first transistor and the second transistor form a complementary structure so that when the data line uses the first transistor to drive the OLED device, the second transistor is in the OFF state, causing no power consumption. When the data line is in the LOW state, the first transistor is in the OFF state. The second transistor is in a sub-critical state after getting rid of extra charges. Therefore, the only power loss in the whole circuit is due to the leakage current of the first transistor. The power loss is in the order of pico-watts.
The first transistor and the second transistor proposed herein can be replaced by an NPN transistor, a PNP transistor, an NMOS or a PMOS.
The driver disclosed herein can be accompanied by a resistor so as to linearly control the voltage. The resistor can be replaced by an active transistor load.